1. Field of the Invention
The invention relates to data hiding, and more particularly, to data hiding methods and corresponding systems for hiding secret data into two or more halftone images using an adaptive noise-balanced error diffusion (ANBEDF) technique with moderate computational complexity.
2. Description of the Related Art
Digital halftoning is a process to display grayscale images with a two-tone texture pattern. Halftoning is mainly used as printouts for materials such as magazines, newspapers, and books, generating a black-and-white format. Halftoning mainly takes advantage of the fact that the human visual system is not highly sensitive, so that black and white pixels of a dense uniform grid may be used to represent a desired grayscale effect. Halftoning techniques can be mainly divided into two categories: single pixel processing, and neighboring pixel processing. For single pixel processing, a halftoning output can be obtained by comparing pixel values of every pixel of an original image with some masks. Ordered dithering, for example, is a well-known scheme in the single pixel processing field. For neighboring pixel processing, however, the halftoning output can not be simply obtained by comparing pixel values and a further filtering process is required. Error diffusion, for example, is a well-known scheme in the neighboring pixel processing field. Error diffusion produces good image quality and reasonable computational complexity. Therefore, error diffusion is typically adopted for embedding secret patterns in order to obtain better image quality of the halftoning output image.
Recently, transmittance of digital information over the Internet has rapidly grown. Digital data may be easily downloaded or manipulated and intentionally tampered with, thus making the issue of intellectual property protection more significant. Most multimedia files may be stored in a compressed bit stream format to save on storage space or transmission time. As a result, methods for data hiding have grown in significance. Embedding digital watermarks or digital signatures in multimedia content is one method of intellectual property protection for digital information, verification of ownership rights and assuring accuracy of digital information.
Hiding data in halftone images using a halftoning technique is commonly used for data hiding. In one technique, secret visual patterns are embedded into two or more halftone images. When implemented, the secret visual patterns can be clearly perceived when the halftone images are overlaid with each other. One method used to embed watermarks into the halftone images is the noise-balanced error diffusion (NBEDF) method. In the NBEDF method, an additive noise is adopted to force the pixels in the embedded images to be either correlative or independent, according to whether the pixel value in the watermark is black or white. The average color shifting error caused by the additive noise is compensated for by adding an equal amount of opposite noise to the neighboring unprocessed pixels. The NBEDF method may be simply implemented and visual decoding via printing an embedded image on transparencies is performed efficiently.
Although good image quality may be provided with the NBEDF method, the decoded pattern quality may significantly degrade such that deckle edge portions of the obtained secret image become visible when the additive noise is small. In addition, has also been observed that areas with different variances may show different degradation qualities, even if the same noise strength is added.